Quick answer
Green water is free-floating algae. Reduce excess light, address nutrient inputs, improve maintenance and consider a properly sized UV unit for persistent cases.
What to check first
- Confirm the water itself is green rather than the glass
- Check direct sunlight
- Review feeding and fertiliser doses
- Test nitrate and phosphate trends
Write down the test results, recent maintenance, new livestock, feeding changes and equipment faults. A short timeline prevents repeated guesses and makes patterns easier to see.
A practical action plan
- 1. Black out only when livestock and plants can tolerate itRecord what changes and retest before making another major adjustment.
- 2. Shorten the photoperiodRecord what changes and retest before making another major adjustment.
- 3. Perform sensible water changesRecord what changes and retest before making another major adjustment.
- 4. Use fine filtration or UV where appropriateRecord what changes and retest before making another major adjustment.
What to avoid
Do not use repeated algaecide doses without addressing the cause.
When it is urgent
A sudden die-off of algae can reduce oxygen; watch livestock closely.
How to prevent it next time
Use stable routines: quarantine additions, test before and after major changes, maintain backup aeration, keep equipment accessible, and record enough detail to compare today with the aquarium’s normal baseline. Prevention is usually cheaper and safer than emergency correction.
Research and review
This is original AquaNexus writing prepared for practical aquarium use. The husbandry framework was checked against authoritative veterinary, safety, scientific or specialist references.
Test rather than guess and make changes at a pace the livestock can tolerate. This guide does not replace aquatic veterinary advice. Follow product labels and local electrical-safety requirements.